Hydraulic frictional coupling

ABSTRACT

In the field of fastening components together in such a way as to be able to transmit mechanical power, keyways and keys have been used; also, interlocking conical tapers have been used which are actuated either by a ring of screws or by oil-injection. 
     The coupling according to the invention comprises an annular chamber housing an annular piston; the chamber and the piston have matching tapers. Oil is pumped into one end of the chamber to move the piston axially in one direction to cause radial expansion of the chamber to connect two components; oil is pumped into the other end of the chamber to move the piston in the opposite direction to disconnect said components. 
     Uses of the coupling are, for example, to join the adjacent ends of coaxial shafts and to act as a locking bush to join concentrically arranged components.

BACKGROUND OF THE INVENTION.

1. Field of the Invention

This invention relates to frictional locking couplings used, forexample, in mechanical power transmission systems to fasten two shaftstogether, or to fasten two components to one another in such a manner asto resist axial thrust, or even a combination of the two.

2. Description of the Prior Art

For many years, in the field of mechanical power transmission, the useof keys and keyways has been accepted practice, although the severestress concentration in shafts due to keyseats has long been recognizedas one of the prime causes of failure due to fatigue. Furthermore,press-fitting is often employed in conjunction with a key and thisaccentuates the problems of mounting and dismounting whilst compoundingthe effect of stress concentration.

In recent years several types of devices have become available whichovercome some of the problems outlined above. Most of these devices useinterlocking conical tapers which are actuated by means of a ring ofscrews; these screws must all be tightened in a correct sequence and toa specified torque setting to ensure satisfactory torque-transmittingcapacity on the part of the joined components. For small-diameter shaftsand light-duty joints this may well be acceptable; however, on largeheavy-duty applications the process of tightening the screws becomesextremely tedious and time-consuming. In the course of tightening asingle joint it is not uncommon to apply a torque-wrench ten or twelvetimes to each of perhaps two dozen screws before finally achieving thecondition when all are fully tight. If any one screw is found to requirefurther tightening, then the process of tightening this one screw willalso necessitate going around all the remaining screws because thesewill very probably have become loosened as the conical tapers move.Furthermore, the heads of the screws lie in close proximity to thesurface of the shaft and this makes it difficult to apply thetorque-wrench; in some instances, a special torque-wrench with truncatedhead is necessary to gain access to the screw heads. The time factoralone makes the fitting process very expensive for practical engineeringpurposes.

Another method of achieving a joint which is suitable for thetransmission of mechanical power and which is readily dismountable isthe oil-injection method. This entails injecting oil at very highpressure between mounted components, the oil pressure being slightly inexcess of the surface pressure of the interference fit. To facilitatemanufacture, the mating surfaces are usually slightly tapered. This is avery reliable method, well-proven in heavy engineering over many years.There are, however, several drawbacks. Firstly, the tapered surfaces aredifficult to manufacture accurately with the necessary oil-feed anddrainage ducts. Secondly, the finely tapered surfaces render itimpossible to achieve accurate axial positioning of the mountedcomponents without using an intermediate sleeve. Thirdly, specialexternal hydraulic tooling has to be provided to supply axial thrust tothe joint whilst fitting; normally, two separate hydraulic systems areused, one for radial pressurisation and the other for axialpressurisation.

SUMMARY OF THE INVENTION

This invention provides a device for hydraulic frictional locking of twocomponents to one another which overcomes most of the aforementioneddifficulties and results in a readily mountable and dismountable jointcapable of transmitting high torque and/or resisting axial thrust. Thisis achieved, essentially, by using an annular coupling which comprises acaptive axially displaceable piston. When the piston is displaced in afirst axial direction, the coupling assumes a dilated condition in whichfrictional locking is achieved, and when the piston is displaced in asecond axial direction (opposite to the first axial direction) thecoupling resumes its undilated condition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an axial section through two components joined by a bushaccording to a first embodiment;

FIG. 2 is a view similar to FIG. 1 and in which an alternativeembodiment of the bush is illustrated;

FIG. 3 is an axial section through a third embodiment of a bushingaccording to the invention;

FIGS. 4(a) and 4(b) illustrate developments of the internal/externalsurfaces of two further embodiments of a bush or other type of couplingaccording to the invention; and

FIGS. 5(a) and 5(b) illustrate axial sections through two shafts whichare connected to one another by two different embodiments of a couplingaccording to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring firstly to FIG. 1, there is illustrated therein a bush withcylindrical external surfaces which is mounted coaxially betweencylindrical surfaces of the outer and inner components a, b which are tobe fastened to one another. When hydraulically pressurised from anexternal source, the bush dilates (i.e. the outer diameter increases andthe inner diameter decreases) so as to produce effectively aninterference fit at both outer and inner surfaces; in order to achievethis, the hydraulic frictional locking bush has an inner annular cavityc containing an annular piston d. The piston is an accurate fit radiallywithin the cavity and at least one of the fitting surfaces of each ofthe cavity and piston is conical. The piston can move axially under theinfluence of a pressurised hydraulic fluid which may be admitted toeither end of the piston depending on whether it is desired to mount ordismount the assembled components. The taper of the conical surfacesensures that axial movement of the piston results in radial expansion orcontraction of the bush. All mating surfaces are of accurate form andare assembled with slight interference to overcome leakage, even in therelaxed condition. Alternatively, annular sealing rings such as `O`rings, or similar, may be employed. The taper of the conical surfaces issufficiently fine to render them self-sustaining.

In order to facilitate axial movement of the piston under the very highradial loads, the oil-injection principle (previously briefly described)is utilised to provide an oil-film between the pairs of opposed surfacese and f, where relative movement occurs.

However, it is to be noted that a single source of high-pressure fluid(which may be, for example, a high-pressure hand pump) is used toprovide both the axial and radial forces simultaneously. Oilways throughthe piston provide a supply of hydraulic fluid to said mating surfacese, f regardless of the side from which the annular piston ispressurised, whilst leakage to the non-pressurised side of the piston isprevented by a simple valve arrangement g. The external source ofhydraulic pressure is connected to tapping h for mounting, and totapping i for dismounting. The annular fastener l prevents axialseparation of the component parts of the bush.

The valve arrangement g comprises a rod r and two balls s, t which areadapted to seat against respective valve seats in dependence upon theend from which the cavity or chamber c is pressurised. When hydraulicpressure is applied at tapping h, ball s is moved to the right away fromits seat, displacing rod r against ball t to push ball t against itsvalve seat. This permits the oil applied to piston d to be also conveyedradially between the contacting surfaces of the piston and encirclingcomponents, without passing to the opposite side of the piston.Conversely, when hydraulic pressure is applied through tapping i, ball tis lifted off its valve seat and rod r pushes ball s against its valveseat. Said valve arrangement g can be modified by replacing the rod r bya coiled compression spring (not illustrated) which tends to urge bothballs s,t, onto their respective seats at all times, said spring forcebeong overcome by the pumped hydraulic fluid.

The manufacture of the hollow bush presents practical difficulties insealing against leakage, and also in mechanical strength to resist axialloads imparted by the high hydraulic pressure. FIG. 1 shows a two-partbush which has metal-to-metal skirt-type seals j to overcome the sealingproblem. It is felt that a more effective and economical method ofmanufacture will be to make the bush in three parts and to employ theelectron beam welding process to fuse two co-axial junctions k (FIG. 2)simultaneously at each end of the bush. This method of welding will giveminimal distortion and high strength.

FIG. 3 depicts a practical design of bush (bushing) which is made in twoparts which are joined to each other by electron beam welding to formthe cavity or chamber c for the piston d. The welding junctions m and nmay be assembled with slight interference fit. The weld does notpenetrate right through the junction and therefore the prospectiveproblem of contamination of the cavity by weld debris is obviated. Theprocess of electron beam welding is virtually essential to the successof this design as it gives high strength with minimal distortion.

It is also to be noted that the configuration of the bush shown in FIG.3 yields an important advantage, namely, that the degree of interferencefit achieved is easily monitored and is therefore readily controllable.Thus, during mounting, a depth gauge may be inserted through the vacantdismounting oil tapping hole i to touch the piston d. The axial drive-updistance of the piston thus measured gives a direct indication of theinterference fit induced.

FIG. 3 also does not include the valve arrangement g shown in FIGS. 1and 2. Instead, a pair of non-intersecting helical grooves p and q (FIG.4(a)) are cut on both the external and the internal sliding surfaces ofthe piston. At each sliding surface, one groove of the pair runs rightthrough into (is open at) the respective end face of the piston to befed by oil from the cavity or chamber whilst the other end of saidgroove stops well short of the other end of the piston and therebyprevents leakage of the pressurized oil past the piston. The other ofthe two grooves is arranged in the converse manner. This arrangementgives a closed oil-feed to the sliding surfaces from each end of thepiston, but without the complication of the valve arrangement g. As analternative to the form shown in FIG. 4 (a), the helical grooves maysimply be replaced by several straight axial grooves which are open atone end and closed at the other as shown in FIG. 4 (b); such axiallyextending grooves also give an oil-feed capability without any leakage.Thus, these helical and straight grooves convey oil from the pressurizedface of piston d to between the contacting surfaces of the piston andencircling components. The flow of oil is halted at the closed end ofeach groove such that the flow does not extend through to the oppositeface of the piston and relieve the pressure on the pressurized face.

FIGS. 5(a) and (b) show two different versions of a coupling for joiningtwo shafts to each other, said coupling utilising the principleunderlying the present invention. In FIG. 5(a), the end portions of twocoaxial shafts 10, 11 are shown joined to one another by a couplingindicated generally by the reference numeral 12. The coupling 12consists of two components 13, 14 and an annular piston 15 which ishoused within an annular cavity or chamber 16; the components are joinedto one another at 17, 18 by welds preferably made by the electron beamwelding process. The generally axially extending internal surfaces ofthe chamber 16 are such as to provide a taper and the annular piston hasits corresponding surfaces so formed as to provide a matching taper. Atapping 19 and an associated conduit communicate with one end of thechamber 16 and another tapping 20 and an associated conduit communicatewith the other end of said chamber, the tappings 19, 20 are respectivelythe mounting and dismounting tappings. The component 14 has had itsradial thickness increased considerably when compared with thethicknesses of the corresponding parts in FIGS. 1 to 3; this has beendone in order to ensure that the unit as a whole will be able torestrain or absorb the hoop stresses which result from the applicationof hydraulic pressure by the pump. Further description of thisembodiment is not considered to be necessary.

In FIG. 5(b), the end portions of two coaxial shafts 30, 31 are shownjoined to one another by a two-part coupling which is indicatedgenerally by the reference numeral 32. It will be seen that each part ofthe coupling 32 consists of two components 33, 34 and an annular piston35 which is housed within an annular chamber 36; the chamber and thepiston taper as already described with reference to FIG. 5(a). There arealso tappings/conduits 37, 38 for mounting and dismounting,respectively, and welds 39, 40 as already described with reference toFIG. 5(a). It should be noted that, although only the right-hand part ofthe coupling 32 is illustrated with its tappings/conduits 37,38, theleft-hand part is also equipped therewith and that these have not beenshown for the reason that they are not in the same section plane as theones which have been illustrated.

The right-hand part of the coupling 32 is provided with an axiallyprojecting boss or spigot 41 which (in the assembled conditionillustrated) extends into a corresponding re-entrant recess 42, therebyproviding a centering and interlocking feature. The two parts of thecoupling 32 are bolted together as indicated by the reference numerals43. Further description of this embodiment is also considered to beunnecessary.

The embodiment of FIG. 5(a) is suitable in locations in which, when forrepair or for maintenance or for some other reason, the separation ofthe two shaft portions 10, 11 becomes necessary. In such a case, thecoupling 12 is dismounted by appropriate operation as hereinbeforedescribed and the slackened coupling is moved to the right or to theleft so as to clear the respective shaft end portion. In cases wherethere is little or no room in one direction or the other for such axialsliding of the coupling along the components, the embodiment of FIG.5(b) is suitable because one need only undo and remove the bolts/nuts 43in order to be able to separate the halves of the coupling 32 axiallyjust far enough to disengage the boss 41/recess 42; the FIG. 5(b)embodiment would be very useful, for example, in a coal mine where spaceis at a premium and in which for example drive heads (namely, thecouplings between a gear box and the driving drum of a conveyor) need tobe serviced.

The sliding action which is required of the annular piston in each ofthe embodiments described above with reference to the drawings isfacilitated by making the piston of one material (for example, cast ironor bronze) and by making the two components which define the internalchamber of steel. This is not to say that steel for both parts would notwork, especially in the big sizes but it is considered to be preferableto use dissimilar metals. In general, also it is preferred to avoid theuse of any lubricant on the piston because of the welding which needs tobe carried out after assembly of the piston and the two chamber-formingcomponents.

It will be evident to engineers that the invention not only can providea connection between two components for the purpose of the transmissionof motive power but also can provide a connection between two componentssuch as will resist axial thrust; the connection could also cope with acombination of axial thrust and torque.

Some advantages which stem from the use of a coupling according to thepresent invention are as follows:

(1) A torque wrench, previously required for the tightening of the ringof screws, is not required. A compact high-pressure hand pump necessaryto mount and dismount the coupling is not more expensive than the torquewrench but the time saved in fitting the coupling leads to considerableeconomies.

(2) The amount of time which can be saved is exemplified by thefollowing:-

Tests with a locking bush for a shaft of 100 mm diameter (firstcomponent) and a second component having a bore of 145 mm diameter havegiven the following results:-

Time to mount=5 minutes, approx.

Time to dismount=3 minutes, approx.

Slip torque=21000 N.m. (83 U.K. ton inches).

(3) The torque capacity and/or the resistance to axial thrust can besignificantly increased by degreasing the components before assembly.

(4) The ability to transmit torque and/or to resist axial thrust is notdependent on retaining oil at high pressure; the oil is drained outafter use.

(5) The fine taper of the chamber and of the enclosed piston is suchthat the parts are infinitely self-sustaining.

(6) Whereas (with one known coupling which employs the oil-injection)dismounting can cause the outer one of the two tapered components tomove axially along the coupled shafts very suddenly, with the resultthat people normally stand well clear of the coupled shafts, thecoupling according to the present invention releases progressively andnonexplosively. When carrying out dismounting, there is a build-up ofhydraulic pressure until the first movement of the annular piston takesplace; this is detected by a drop in pressure at the pump and thepumping is continued at ever-decreasing pressure until the piston hasbeen moved back to its end position.

(7) The high torque capacity and/or high resistance to axial thrust isassured (especially with degreasing as in (3) above) because theexternal surfaces of the coupling are not oil-wetted.

(8) The coupling is easily dismounted and easily re-used.

(9) The degree of interference fit obtained by the mounted coupling canbe easily monitored and precisely controlled.

(10) The coupling can be used in locations where accessibility is poorbecause the high-pressure hand pump can be operated from a distance vialengthy connecting hoses.

(11) Mounting of the coupling is reliable and consistent; tighteningoccurs evenly without skewing and without the uncertain torque-frictioncharacteristics of screws.

(12) There is no tendency of the coupling or of the clamped component(s)to make any axial movement during mounting or dismounting.

(13) The relative angular positions of components and coupling may beadjusted infinitely and yet there is zero backlash once the coupling hasbeen mounted.

(14) The coupling has excellent self-centering characteristics.

(15) The coupling is tamperproof.

(16) The coupling has a low susceptibility to contamination because its"sealed unit" construction (the respective mounting and dismountingtappings are, of course, plugged after use to keep out foreign bodies)requires only the use of clean mineral oil.

(17) There is improved shaft strength even though the dimensions of theshaft can be reduced; this improved strength stems from the eliminationof keyways, splines, cotters and so on. Moreover, there is improvedfatigue life.

(18) The elimination of heavily loaded screws gives an enormousreduction in both assembly time and effort. Moreover, operators do nothave to handle heavy tooling.

(19) The coupling is capable of acting, in the last resort, as anoverload protection of expensive machine components against damage andthis is something that keyed components could never provide. Thecoupling is not, however, to be used repeatedly as a form of slippingclutch.

(20) The hydraulic coupling, being mounted so simply and quickly, lendsitself to an assembly line or moving track. Thus, for example, thecoupling could be used where it is necessary for a flywheel to bemounted on an engine shaft; this could be done quickly and efficientlyby means of an electric pump or by means of a pump which is driven bycompressed air.

(21) It must be stressed that, in order to mount/dismount a couplingaccording to the invention, a single source of high-pressure fluid isused; this is an advantage when compared with at least one knownarrangement which employs high pressure injectors to create thenecessary oil film between the respective sleeves and a low pressurepump to cause relative axial movement between said sleeves.

It is considered that the field of application is extremely wide but thefollowing are mentioned by way of example:-

Machine tools. Drive shafts, cranks, gears and flywheels.

Marine uses. Propellers, couplings, rudder stocks.

Mining equipment. Drum endplates, crusher rotors, gearbox couplings,cutting heads, winch drums.

Process machinery. Valve stems, sprockets, levers, cams, papermachinery.

I claim:
 1. A coupling for frictionally joining and locking twocomponents for simultaneous angular and axial movement, comprising:firstand second concentric annular members connected by liquid-tight seals;at least one annular chamber defined by corresponding surfaces of saidannular members; at least one captive annular piston housed within saidchamber, said chamber and said piston having matching axially extending,conically tapered surfaces; injection means for selectively supplyinghydraulic fluid under pressure to opposite ends of said piston andselectively causing radial expansion and contraction of said chamber byaxial displacement of said piston.
 2. A coupling according to claim 1wherein said annular members and said piston form a locking bush havingonly one annular piston, said bush including conduit and valve means forconveying liquid pressurizing one end of said piston to between saidpiston and said annular members and for producing a film of the liquidbetween contacting surfaces thereof to facilitate axial movement of saidpiston in said chamber.
 3. A coupling according to claim 1 wherein saidannular members and said piston form a frictional locking bush havingonly one piston, said piston including first and second nonintersectinggrooves in each of axially extending, radially inwardly and outwardlyfacing surfaces of said piston, each of said first grooves opening atone end of said piston into said chamber, each of said second groovesopening at an opposite end of said piston into said chamber, each ofsaid grooves having a closed end intermediate the ends of saidpiston;whereby liquid supplied to an end of said piston produces a filmof the liquid between contacting surfaces of said piston and saidannular members facilitating axial movement of the piston.
 4. A couplingaccording to claim 3 wherein said nonintersecting grooves are helical.5. A coupling according to claim 3 wherein said piston comprises aplurality of said non-intersecting grooves arranged in two sets, saidgrooves extending axially relative to said bush.
 6. A coupling accordingto claim 1 wherein said annular members form a sleeve housing a singlepiston, said second annular member being radially outside said firstannular member and having a radial thickness greater than that of saidfirst annular member for absorbing loop stresses generated by liquidpressure.
 7. A coupling according to claim 1 wherein said second annularmember comprises first and second parts coupled by fastening means forforming a sleeve, each of said parts having a separate chamber housing asingle annular piston and having a separate injection means forsupplying hydraulic fluid under pressure to opposite sides of therespective piston to move the respective piston axially, said first parthaving an axially extending annular re-entrant recess, said second parthaving a complementary axially extending boss mating with said recess.